This is perfectly fine when you are driving some app that has a per-user experience that allows you to wrap up most of their experience in some blobs.

However I would still advise people to use a third normal form - they help you, constraints help you, and often other sets of tooling have poor support for constraints on JSON. Scanning and updating every value because you need to update some subset sucks.

You first point is super valid though - understanding the domain is very useful and you can get easily 10x the performance by designing with proper types involved, but importantly don't just build out the model before devs and customers have a use for anything, this is a classic mistake in my eyes (and then skipping cleanup when that is basically unused.)

If you want to figure out your data model in depth beforehand there's nothing wrong with that... but you will still make tons of mistakes mistakes, lack of planning will require last minute fixes, and the evolution of the product will have your original planning gather dust.